1. Field of the Invention
The present invention relates to a startup circuit, and more particularly to a startup circuit applied in a bandgap voltage generator.
2. Description of the Prior Art
Conventionally, a bandgap voltage generator is utilized for generating a precise voltage and reference voltage, where the voltage should be a fixed voltage that is unaffected by the environment temperature. A startup circuit is coupled to the bandgap voltage generator for activating the bandgap voltage generator. After the bandgap voltage is generated, the startup circuit will be turned off automatically in order to reduce power consumption.
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a prior art startup circuit 110. The startup circuit 110 is utilized in a bandgap voltage generator 100. If an error has occurred in the turn on time and the turn off time in the startup circuit 110, the bandgap voltage generator 100 will not operate properly. For example, if transistor M1 of the startup circuit 110 is turned off (i.e. the voltage at terminal C is smaller than the threshold voltage Vth of the transistor M1), but the BJT transistor Q1 of the bandgap voltage generator 100 is not turned on yet (i.e. the voltage Vin at the terminal A is smaller than the base-emitter voltage Vbe of the transistor Q1), then misjudging of the bandgap voltage generator 100 will occurred. On the other hand, if transistors Q1 and Q2 of the bandgap voltage generator 100 are turned on (i.e. the voltages Vin, Vip at the terminals A, B are larger than the base-emitter Vbe of the transistors Q1 and Q2, respectively), but the transistor M1 of the startup circuit 110 is not turned off (i.e. the voltage at the terminal C is larger than the threshold voltage Vth of the transistor M1), the startup circuit 110 will affect the biasing condition of the bandgap voltage generator 100, in which an error bandgap voltage is generated. Therefore, in order to avoid the above-mentioned problem, the startup circuit 110 should satisfy the following two equations:
                                                        V              DD                        -                                          I                                  M                  ⁢                                                                          ⁢                  3                                            ·                              R                1                                              <                      V            tn                          ,                            (        1        )                                                                    V              be                                      R              2                                +                                                    ln                ⁡                                  (                  n                  )                                            ·                              V                T                                                    R              3                                      >                  I                      M            ⁢                                                  ⁢            3                          >                                            V              be                                      R              2                                .                                    (        2        )            
According to the equations (1) and (2), the resistor R1 and the current IM3 of the startup circuit 110 should be kept within a predetermined range to guarantee the normal operation of the bandgap voltage generator 100. Therefore, the startup circuit 110 should be well designed to conform to the variation of the bandgap voltage generator 100.